Methods and systems for detecting foreign objects in a wireless charging system

ABSTRACT

Methods and systems are described for using detection coils to detect metallic or conductive foreign objects that can interfere with the wireless transfer of power from a power transmitter to a power receiver. In particular, the detection coils are targeted to foreign objects that are smaller than a power transmitter coil in the power transmitter.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of co-pending U.S. patent applicationSer. No. 14/869,597, filed Sep. 29, 2015; which is a continuation ofU.S. patent application Ser. No. 13/628,348, filed Sep. 27, 2012. Thecontents of all of the foregoing is incorporated by reference herein inits entirety.

BACKGROUND

Embodiments of the present disclosure relate generally to wireless powertransfer devices. Specifically, embodiments of the present disclosurerelate to methods and systems for detecting foreign objects that mayinterfere with a wireless charging device.

Wireless power is now widely used for charging mobile devices, chargingelectric vehicles, powering biomedical devices, and other applications.Wireless power transfer is implemented using a power transmitter thattransfers power to a power receiver. The power receiver is oftenintegrated with or attached to the end device being charged by thewireless power transfer system, although the power transmitter devicetypically is not physically attached to the end device. Electronicsconnected to the transmitter transform power from a power source(whether alternating current or direct current) to a suitable form todrive a power transmitter coil in the transmitter. The power is thentransferred from the power transmitter coil to a power receiver coilusing inductive coupling. Electronics in the receiver then condition thepower from the receiver coil, generating suitable output to power thedevice or charge a battery connected to the device.

In an ideal wireless power transfer system, the transmitted power andreceived power are equal, meaning that no power is lost duringtransmission. However, because power is transmitted using anelectromagnetic field, energy can be lost in the system when the fieldinteracts with metal or electrically conductive parts not configured topower or charge the device. Not only does the resulting power loss leadto a reduction in efficiency of the wireless charging system, but it canalso cause heating of the metal parts. This heating can, in turn, damagethe device or pose a threat to the safety of the user.

SUMMARY

Embodiments of the present disclosure include methods and systems forusing one or more foreign object detection coils in a wireless powertransmitter system to detect the presence of a foreign object. Certainembodiments detect foreign objects without needing or receivinginformation from the receiver system regarding the amount of power incomparison to the amount of power transmitted. Some embodiments includea detection coil that is smaller than the power transmitter coil. Thisenables detection of foreign objects that are small, such as coins,rings, and other similarly sized foreign objects.

In some embodiments, one or more detection coils in a coil array areused to detect foreign objects that can interfere with the wirelesstransfer of power from a power transmitter coil to a power receivercoil. As with other embodiments, the coils of the coil array can beconfigured to have a size that is smaller than the power transmittercoil of the wireless power transfer system and preferably comparable tothe size to foreign objects, thereby improving the coupling (andtherefore detection sensitivity) between the foreign object detectioncoil and the foreign object. The coils of the detection array can be ina single layer, or multiple layers that are offset from one another toprovide more thorough detection coverage.

Embodiments also can change the detection distance in the Z-direction(the direction of power transfer) by appropriately sizing the detectioncoil. This enables the detection coils to detect foreign objects withoutincorrectly identifying friendly parasitic components of the devicebeing charged as foreign objects, in cases in which the friendlyparasitic components are further away from an interface between thepower transmitter system and the power receiving system.

Other parameters can also be changed to change the detection distance inthe Z-direction. For example, certain embodiments use a resonant circuitthat includes the detection coil. The behavior of the resonant circuitis used to identify the presence of foreign objects. It is possible toadd a resistor connected in series with the detection coil (or in serieswith a capacitor). By adding a resistor, the quality factor of theresonant circuit is decreased, thereby also decreasing the detectiondistance. In another approach, the detection distance can also beadjusted by changing the frequency of the resonant circuit. Theadjustment of the resonant frequency can be done by changing capacitorvalues in the resonant circuit, for example.

In other embodiments, the detection coils are connected to a locationunit that uses the detection responses from the coils to determine alocation of a foreign object.

In some embodiments, capacitors that are part of a receiver coil circuitcan be charged prior to using foreign object detection coils. By“pre-charging” these capacitors, the energy absorbed by the receivercoils during the initial operation of the foreign object detection coilsis reduced. This reduces the likelihood that the foreign objectdetection systems and methods will mistakenly identify the receiver coiland circuit as a foreign object.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration of a wireless power transmission system, inthe absence of foreign objects.

FIG. 2(a) is a circuit diagram of a foreign object detection sensor in awireless power transmission system without a foreign object, and anaccompanying waveform, in an embodiment.

FIG. 2(b) is a circuit diagram of a foreign object detection sensor in awireless power transmission system with a foreign object, and anaccompanying waveform, in an embodiment.

FIG. 3(a) is a circuit diagram of a foreign object detection sensor in awireless power transmission system, and an accompanying waveform, in anembodiment.

FIG. 3(b) is a circuit diagram of a foreign object detection sensor in awireless power transmission system, and an accompanying waveform, in anembodiment.

FIG. 4(a) is a cross-sectional view of a wireless power transmissionsystem that includes representative friendly parasitic component andforeign object, in an embodiment.

FIG. 4(b) is an exploded view of a wireless power transmission systemthat includes representative friendly parasitic component and foreignobject, in an embodiment.

FIG. 5 is an illustration of a transmitter coil of a wireless powertransmission system and an array of foreign object detection coils, inan embodiment.

FIG. 6 is a graph illustrating coupling between a foreign objectdetection coil and a foreign object as a function of detection coilradius for an assumed foreign object radius of 10 mm, in an embodiment.

FIG. 7 is a graph depicting electromagnetic coupling between a foreignobject detection coil and a foreign object as a function of Z-axisdistance from the detection coil for two different radius (or diameter)coils, in an embodiment.

FIG. 8(a) is a plan view of a foreign object coil detection array,wherein the array is a single layer of detection coils, in anembodiment.

FIG. 8(b) is a plan view of a foreign object coil detection array,wherein the array is a double layer of detection coils, the second layerhaving an offset from the first layer in the lateral direction, in anembodiment.

FIG. 9(a) is a plan view of two overlapping power transmitter coilsdefining three active areas, a foreign object detection coils disposedin each of the three active areas, in an embodiment.

FIG. 9(b) is a plan view of two overlapping power transmitter coilsdefining three active areas, two foreign object detection coils disposedoverlapping a third detection coil in the active area formed by theoverlapping transmitter coils, in an embodiment.

FIG. 10 is a graph showing a change of Q2/Q1 with the change of the Q1due to adding a resistor to a resonant circuit, in an embodiment.

FIG. 11 is an illustration of the change of Q2/Q1 with the change ofoscillating frequency, thereby reducing the detection distance of adetection coil, in an embodiment.

FIG. 12(a) is a circuit diagram of a receiver circuit used in areceiver, in an embodiment.

FIG. 12(b) is a circuit diagram of the receiver circuit of FIG. 12(a),also showing the parasitic capacitance of the rectifier diodes, in anembodiment.

FIGS. 13(a)-(c) illustrate various voltage and current characteristicsof a foreign object detection resonant circuit, in an embodiment.

FIG. 14 is a method flow diagram of an example algorithm used by aforeign object detection circuit to detect a foreign object, in anembodiment.

The figures depict various embodiments of the present invention forpurposes of illustration only. One skilled in the art will readilyrecognize from the following discussion that alternative embodiments ofthe structures and methods illustrated herein may be employed withoutdeparting from the principles of the present disclosure.

DETAILED DESCRIPTION Overview

Embodiments described in the present disclosure include methods andsystems for detecting metallic or conductive foreign objects (“foreignobjects” for brevity) that can interfere with the wireless transfer ofpower from a power transmitter to a power receiver. In particular,foreign objects disposed between the power transmitter and the powerreceiver absorb some or all of the transmitted energy, thereby reducingthe efficiency of the wireless charging device and/or posing a safetyconcern by becoming hot. This is particularly problematic for smallerforeign objects that absorb some energy, but not enough to trigger afault terminating the power transfer. Also, the methods and systemsdescribed can be implemented inside a transmitter and work without anyinformation from the receiver.

Foreign Object Detection Techniques

There are two broad types of foreign objects that can absorb energytransmitted by a wireless charging device. The first type includesconductive components or objects that are part of the device beingpowered or charged by the wireless charging device. These conductivecomponents of the device are often called “friendly parasiticcomponents.” The second type includes conductive parts or objects thatare not part of the device being charged. These are often referred to as“foreign objects.”

For devices designed to accommodate wireless charging, friendlyparasitic components are often configured to cause reduced or negligiblesafety issues. Examples of such configurations include, but are notlimited to placing the friendly parasitic components away from the mostintense areas of the electromagnetic field emitted by the transmitter,shielding the friendly parasitic components with a shielding layer thatreduces the energy absorbed by the component, designing the friendlyparasitic component to accommodate, absorb, or exhaust the thermalenergy induced in the component by the transmitted energy, and/or othertechniques. Furthermore, when any of these configurations are used, thefriendly parasitic component will generally not be detected as a foreignobject by embodiments of the present disclosure.

FIG. 1 is an illustration of a wireless charging system 100, in theabsence of foreign objects. In this example, the wireless chargingsystem includes a transmitter system 102, which includes a power source104, transmitter electronics 108, and a power transmitter coil 112. Thesystem 100 also includes a receiver system 114, which includes a powerreceiver coil 116, and receiver electronics 120. An interface 124separates the transmitter system 102 from the receiver system 114.

In the system 100, the power source 104 supplies the transmitterelectronics 108 with power. The power can be alternating current (AC) ordirect current (DC). Regardless, the transmitter electronics 108conditions the power so that the power transmitter coil 112 receives anelectrical current. The electrical current in the transmitter coil 112produces an electromagnetic field. This electromagnetic field isrepresented by the “transmitted power” and the “received power” arrowsin FIG. 1. This electromagnetic field is inductively coupled into thepower receiver coil 116.

In this way, power is transferred across the interface 124 disposedbetween the power transmitter coil 112 and the power receiver coil 116.In some examples, the interface 124 is the interface between a housingthat encases the transmitter coil 112 and, optionally, another housingthat encases the receiver coil 116. Depending on the nature of theinterface 124, the interface can absorb or otherwise attenuate thetransmitted power so that the transmitted power is greater than thereceived power. Regardless of the reason, the power that crosses theinterface 124 and reaches the power receiver coil 116, as shown by thereceived power arrow in FIG. 1, can be less than the transmitted power.Foreign objects located in the power transmission path can disturb thepower transfer.

Foreign objects can be detected using the principle of decay in aresonant circuit, as shown in FIGS. 2(a) and 2(b) and as described inU.S. Pat. No. 4,843,259, which is incorporated by reference herein inits entirety.

The basic principle of using decay in a resonant circuit for foreignobject detection is explained as follows. As shown in FIG. 2(a), theforeign object sensor 200 includes a detection coil 204 and a capacitor208, which together form a resonant tank circuit 200. The resonant tankcircuit is charged by an external power source. After the external powersource is removed, the energy stored in the resonant tank circuit 200will oscillate between the detection coil 204 and the capacitor 208. Theenergy will decay with time due to the power dissipation within thedetection coil, the capacitor, and any other parasitic components (e.g.PCB-traces or wires). As the power dissipation in the resonant circuit200 itself is normally low (if the detection coil and the capacitor bothhave low internal resistance), the amplitude of the oscillation isrelatively high and the duration of the oscillation is relatively long,as shown in the accompanying waveform 212. In this situation, thequality factor (Q) of the resonant tank circuit is high and will bereferred to as Q1.

On the other hand, as illustrated in FIG. 2(b), when a metal object isproximately disposed to the detection coil 204 and is inside theelectromagnetic field generated by the detection coil, an eddy currentis induced in the metal object. This adds a complex impedance to thecircuit and is modeled as component ‘R+jX’ 210. In this ‘R+jX’ 210component, ‘R’ represents the power loss in the metal object, while ‘jX’represents the change of the inductance of the detection coil 204 causedby the metal object. ‘X’ can be positive if the metal object isdominantly ferrous, and can also be negative if the metal object isdominantly nonferrous. Normally with the addition of ‘R’ into theresonant tank circuit 200, the quality factor (Q) of the circuitdecreases to a lower value, Q2. In this situation, the oscillationamplitude is relatively lower and the oscillation duration is relativelyshorter, as illustrated in accompanying waveform 216. That is, theresponse of the resonant tank circuit decays more quickly, compared towhen there is no foreign object. The level of the reduction of Q or theratio between Q2 and Q1 (Q2/Q1) depends on, for example, the materialand size of the metal object, the coupling between the detection coiland the metal object, and the oscillation frequency.

The coupling between the detection coil and the metal object isdetermined by the relative position of the metal object to the detectioncoil, and the size difference between them. Normally, a larger reductionin Q or a lower value of Q2/Q1 will make the object easier to detect. Inone implementation, the change in Q can be detected by comparing theoscillation shown in FIG. 2(a) to the oscillation shown in FIG. 2(b).There are other methods to detect the change of Q which can also beused.

FIGS. 3(a) and 3(b) are circuit diagrams showing alternate embodimentsof a foreign object detection sensor based on the resonant tank circuitshown in FIGS. 2(a) and 2(b). Waveforms 320 and 344 are also showncorresponding to the different electrical performance of the circuits.

In FIG. 3(a) an AC power source 304 energizes the resonant tank circuit324, whereas in FIG. 3(b) a DC power source 328 energizes the resonanttank circuit 324. In FIG. 3(a), when the switch 308 is closed, the ACpower source 304 energizes the resonant tank circuit 324, resulting in aconstant amplitude oscillation as shown on the lefthand side of thewaveform 320. Once the switch 308 is open, the resonant tank circuit 324oscillates in an undriven condition, decaying over time as shown on therighthand side of wavefront 320. When a foreign object is present, theresponse of the resonant tank circuit 324 decays more quickly, asdescribed in FIGS. 2(a) and 2(b).

In FIG. 3(b), when switch 332 is closed and switch 336 is open, a DCpower source 328 charges a capacitor 340 to a DC value, as shown on thelefthand side of waveform 344. Once switch 332 is open and switch 336 isclosed, the resonant circuit 324 oscillates and decays in an undrivencondition.

In both FIGS. 3(a) and 3(b), by measuring the change of the decay in theoscillation, a foreign object can be detected. However, the decay in theoscillation is not only changed by the foreign object, but also by twoother factors that preferably are not to be ignored in a practicalimplementation. The first factor is the power dissipation in thereceiver circuit, and the second factor is the power dissipation infriendly parasitic components in the device.

Using a Power Transmitter Coil for Detecting Foreign Objects

Embodiments of the present disclosure include systems and methods for awireless power transmitter to detect the presence of a foreign object,preferably without needing or receiving information from the receiver.

FIGS. 4(a) and 4(b) show a cross-sectional view and an exploded view,respectively, of an embodiment of a wireless power transfer system 400.On the transmitter side, the system 400 includes transmitter shielding404 and a power transmitter coil 408. On the receiver side, the system400 includes a power receiver coil 412, and receiver shielding 416. Aninterface 414 separates the transmitter coil 408 from the receiver coil412. A foreign object 420 and a friendly parasitic component 424 arealso shown. Although FIG. 4 shows the components of the wireless powerarranged in a vertical stack for convenience of description, otherembodiments not shown need not be configured in this way.

As shown in FIGS. 4(a) and 4(b), the wireless power transfer systemoften includes a friendly parasitic component 424. The friendlyparasitic components in the system 400 can be directly attached to, orvery close to, the receiver shielding 416. Possible sources of friendlyparasitic components 424 include, but are not limited to, a battery pack(which includes a variety of metallic materials), printed circuit boardwiring, and inter-component wiring. In a typical system configuration,through a magnetic field or electromagnetic field generated by thetransmitter coil 408, power is transferred from the power transmittercoil 408 to the power receiver coil 416 (the power represented by arrowP1 in FIG. 4(a)), as well as partly to the friendly parasitic component424 (the power represented by arrow P2 in FIG. 4(a)). It can be seenthat the power transfer of P2 is mainly due to interaction between theelectromagnetic field with the friendly parasitic components 424. Thereare two reasons that the energy represented by arrow P2 leaks from thesystem, thereby decreasing the efficiency of the wireless power transfersystem 400.

First, the transmitter coil 408 and receiver coil 412, and theircorresponding shielding 404 and 416, are usually not matched indimensions. In many embodiments, the transmitter coil 408 is designed tobe larger than the receiver coil 412 and the receiver shielding 416,such that the receiver coil has a wider range of acceptable charginglocations with respect to the transmitter coil 408. Second, the receivershielding 416, as shown in FIG. 4(a), typically does not completelyisolate the friendly parasitic component 424 from the field P2 forpractical reasons, such as cost.

The third part of the field, represented by the arrow P3, and thecorresponding induced power goes into the foreign object 420. When theforeign object 420 is metal (e.g. a ring or a coin) and positionedwithin this field, an eddy current will be induced inside the metalobject. Electromagnetic energy will be converted into electrical powerloss. The metal foreign object 420 will dissipate this electrical powerby becoming hot.

One way of applying the previously described oscillation and decaymethod to detect foreign objects in a wireless power transfer system isto use the transmitter coil 408 shown in FIGS. 4(a) and 4(b) as aforeign object detection coil. However, when the energy oscillatesbetween the transmitter coil 408 and the added capacitor (shown in FIGS.2(a) and 2(b) as capacitor 208), the field generated by the transmittercoil not only induces power loss in the targeted foreign object 420, butalso in, for example, the friendly parasitic components 424. In otherwords, referring again to FIG. 2(b), the R+jX' component will includeall these factors, which cannot be easily separated.

Using Detector Coils for Foreign Object Detection

In many cases, the foreign object can be assumed to be smaller than thefriendly parasitic components 424 and/or the power transmitter coil 408.Foreign objects that are larger can be detected by other means, forexample large foreign object typically will easily block most of thefield or prevent or significantly reduce coupling between the powertransmitter coil 408 and the power receiver coil 412, which then cantrigger an automatic restriction or termination of the power transfer.Therefore, large foreign objects are less of a concern for this type offoreign object detection. The foreign objects that are more relevant todetect using these approaches are those that are small, andunintentionally positioned at, for example, the interface surface,thereby being exposed to the field. Examples of such objects can be acoin or a ring.

Applying these assumptions to the configuration shown in FIGS. 4(a) and4(b), the power transmitter coil 408 can be larger than the targetedforeign object 420, while having similar size with the friendlyparasitic component 424. In other words, because the transmitter coilhas a size comparable to the friendly parasitic material and larger thana small foreign object, the transmitter coil has better coupling withthe friendly parasitic component than with the foreign object. In thisconfiguration, a detection system that uses the power transmitter coil408 as a detection coil becomes a friendly parasitic component detectorinstead of a foreign object detector.

FIG. 5 illustrates one embodiment of the present disclosure, in which aforeign object detection system 500 includes an array of foreign objectdetection (FOD) coils 504A-G (referred to as 504 for brevity). The coils504 can be configured to have a size smaller than the power transmittercoil 508 and a comparable size to a foreign object 520, therebyimproving the coupling (and therefore detection sensitivity) between theforeign object detection coil 504 and the foreign object 520. Tohighlight this configuration, FIG. 5 shows the transmitter coil 508, thedetection coil array 504 and the foreign object 520, and omits othercomponents of the system. As can be seen from FIG. 5, each detectioncoil 504 has a similar size with the targeted foreign object 520.Matching dimensions enables good coupling between them, and the foreignobject can be better detected.

“Smaller” can be defined in a number of ways. For example, it may bedefined by a lateral dimension appropriate to the shape of the FOD coil.For circular coils, the lateral dimension is a radius or diameter; forrectangular or square coils, the dimension can be a diagonal dimensionor a length of a side. In other examples, the dimension can include awidth, a maximum width, or an area. The lateral dimension may alsocharacterize an FOD coil that is not necessarily planar: solenoidal orcylindrical coils can be characterized by a lateral dimension of onerevolution of the coil and/or a length of the coil. If the FOD coils andpower transmitter coils are all circular, the FOD coil may have asmaller radius than the power transmitter coil. Other standards may alsobe used. For example, the transmitter coil lateral dimension (width,area) may be twice or more times larger than the FOD coil lateraldimension.

FIG. 6 illustrates a simulation of the operation of the system 500 usedto detect a small foreign object in a wireless charging system. Intaking the results of FIG. 6, a steel disc with the radius of 10mm isused to represent many kinds of foreign object 520, such as a coin, abattery, a ring, etc. The x-axis in FIG. 6 represents the increasingradius of the detection coil (e.g., 504), while the y-axis representsthe ratio between Q2 and Q1 (i.e., Q2/Q1). This ratio is introduced inFIG. 2 and needs no further description here. As explained above, alower value of Q2/Q1 is better for detecting foreign objects. As shownin FIG. 6, when the radius of the detection coil is close to the radiusof the disc, the ratio is the lowest and therefore the ability to detectthe foreign object is optimal.

Another advantage of using detection coils 504 that are approximatelythe size of the foreign object 520 and smaller than the transmitter coil508 is the shorter detection distance for the smaller coils,particularly in the Z-direction (the direction of power transfer,perpendicular to interface 414). Referring again to FIG. 4(a), theforeign object is often on or very close to the interface 414 in theZ-direction, while the friendly parasitic components 424 are furtheraway from the interface. It is therefore beneficial to have thedetection coil 504 sensitive to foreign objects 520 that are close tothe interface 414, but insensitive to any objects that are further awayfrom the interface.

FIG. 7 illustrates the comparison of the difference in detection depthof different sized coils. The results shown in FIG. 7 are for asituation in which a steel disc with the radius of 10 mm is chosen asthe representative foreign object. Two detection coils, one with radiusof 5 mm and the other with 14 mm, are tested and compared. The x-axisrepresents the distance between the detection coil 504 and the foreignobject 520, while the y-axis represents the value of Q2/Q1. As shown inFIG. 7, with the small detection coil, the value of Q2/Q1 increases muchfaster. In other words, the detection capability of such small detectioncoils diminishes faster with the increase of distance than a detectioncoil with larger radius. Therefore, for the purpose of detecting foreignobjects, rather than friendly parasitic components located at furtherdistance from the transmitter, smaller coils provide fewer falsepositive results because they are less likely to detect friendlyparasitic components.

Another advantage of using an array of multiple small detection coils isthat an array can be used to determine the location of the foreignobject 520. The location can be determined by comparing the responses ofeach of the detection coils 504 in the array. Since the detection coilsare smaller than the power transmitter coils, the location of theforeign objects can be determined with an accuracy that is better thanthe size of the power transmitter coils.

In some embodiments, the location of the foreign object is detectedusing an array of (partly) overlapping FOD coils. In other embodiments,using this location information, the transmitter can select from amongseveral power transmitter coils to transmit power, thereby redirectingthe field to prevent or reduce power transfer to the foreign object. Inanother embodiment, instead of adding an array of detection coils, onlyone detection coil or a small number of detection coils (fewer thanthose shown in FIG. 5) is placed at locations where the transmittedfield is expected to be the strongest. For example, the detection coilsmay be concentrated in the center of the power transmitter coils.

Foreign object detection coils, such as coils 504 shown in FIG. 5, canbe fabricated by coils wound with wires, by printed circuit board (PCB)coils, by flexible PCB coils, or in other suitable forms. For examplesin which the transmitter coil is fabricated using a PCB, then thedetection coil array can also be fabricated in the same PCB as thetransmitter coil, but detection coils might be fabricated usingdifferent PCB layers from the transmitter coils. Although FIG. 5 showsthat the foreign object detection coil array is on top of the powertransmitter coil, the detection coil array can just as well be disposedbelow the transmitter coils or between multiple transmitter coils.

As mentioned above, the coupling between the targeted foreign object andthe detection coil is dependent on the Z-distance and the relative sizeof the detection coil and the foreign object. The coupling is also afunction of the position of the foreign object relative to the detectioncoil in the lateral direction (that is, in the x-y plane parallel to theinterface 414), due to the shape of the electromagnetic field.

FIGS. 8(a) and 8(b) illustrate another embodiment in which, instead ofusing a single layer detection coil array, a multilayer detection coilmatrix can be used for more uniform detection capability over the wholeinterface area. For simplicity, only the detection coil array is shownin FIGS. 8(a) and 8(b). As shown in FIG. 8(a), the single layerdetection coil array 800 has some detection peaks (marked as ‘P’)corresponding in this plan view to the center of each detection coil804, and some detection valleys (marked as ‘V’) corresponding in thisplan view to the location where the detection coils are less effective(lower coupling). When a foreign object is placed at a locationcorresponding to a ‘V’, it might not be detected. One solution is theuse of multilayer detection coil array 808 as shown in FIG. 8(b). Inthis embodiment, the thick circles represent the coils 804A in a firstlayer, as shown in FIG. 8(a), and the thin circles represent the coils804B in a second layer. In the lateral (x-y) direction, the two layersare overlapping but with an offset. The coils 812 in the second layercompensate the ‘V’s in FIG. 8(a), and result in more uniform detectioncapability. Other patterns are also possible. For example, the there maybe three layers where each layer is hexagonally packed. Alternately,there may be four layers that are square packed but centered at (0,0),(0,L/2), (L/2,0) and (L/2,L/2), respectively, where L is the length ofthe square that defines the square packing. Another embodiment caninclude coils that are not circular but are elliptic, hexagonal, or anyother shape that encloses a particular ‘sensing area’.

In another embodiment, the transmitter system uses more than one powertransmitter coil. The power transmitter coils can also be arrayed, asdescribed above with respect to the FOD coils. As shown in the examplesof FIGS. 9(a) and 9(b), the two power transmitter coils 908A and 908Bare configured in a multilayer structure, overlapping in a centerportion 912. Although FIG. 9 only shows two transmitter coils 908, otherembodiments of the system include more than two power transmitter coils.The system also includes three foreign object detector coils 904A, 904B,904C that are positioned within each of the three regions defined by thetwo power transmitter coils 908, corresponding to operational conditionsin which one or both of the power transmitter coils 908 are active.

In the example shown in FIG. 9(a), the three detection coils 904A, 904B,904C are distributed in one layer. As shown in FIG. 9(b), detectioncoils 904A, 904B, 904C are configured in multiple layers. The foreignobject detection coils 904A, 904B, 904C can be selected for use based onthe receiver coil position and the excitation of one or more selectedtransmitter coils 908. For example, if transmitter coil 908A is excited,only detection coils 904A and 904B will be used to detect foreignobjects. If transmitter coil 908B is excited, only detection coils 904Band 904C will be used to detect foreign objects. If both transmittercoils 908A and 908B are excited, it is optional to check only detectioncoil 904B to determine the presence of foreign objects in the area withstrongest electromagnetic field from the transmitter coils 908A and908B. Optionally, all detection coils 904A, 904B, 904C can be used todetect foreign objects to protect the whole area covered by thetransmitter coils 908A and 908B. The advantage of such ‘localized’foreign object detection is clear. By dynamically localizing the foreignobject detection coil with the position of the active (excited)transmitter coil, the foreign object detection coil concentrates itsdetection inside the region where the field is.

Power transmitter coils 908 can also be activated depending on whichdetections coils 904 detect foreign objects. For example, referring toFIG. 9(a), assume that the power receiver coil is located above thecenter portion 912 so that either power transmitter coil 908A or 908Bcould be used to transfer power to the receiver. If detection coil 904Cdetects a foreign object while detection coil 904A does not, then acontroller may decide to activate only power transmitter coil 908A andnot 908B.

Adjusting Detection Depth of the Detection Coil

In addition to the above embodiments in which the detection distance inthe Z-direction can be adjusted according to detection coil size, otherparameters can also be changed to adjust the detection distance in theZ-direction. For example, it is possible to add a resistor connected inseries with the detection coil (or in series with the capacitor) in thecircuits shown in FIG. 2(a) and FIG. 2(b). FIG. 10 shows the change ofQ2/Q1 with the change of the Q1. By adding the resistor, the qualityfactor of the resonant tank circuit is decreased. That is, low values ofQ1 along the x-axis correspond to high values of added resistance. Asseen from FIG. 10, the value of Q2/Q1 increases with decreasing Ql. Inother words, if the quality factor of the resonant circuit is decreased(by adding more resistance), the detection distance is reduced. Inanother approach, the detection distance can also be adjusted with thechange of oscillating frequency. It should be noted that the oscillatingfrequency (i.e., resonant frequency) of the resonant tank circuit isdetermined by the values of the detection coil load (L) and thecapacitance of the capacitor (C), and is equal to

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The adjustment of the oscillating frequency can be done by changing thecapacitor values. FIG. 11 shows the change of Q2/Q1 with the change ofoscillating frequency. It can be seen from FIG. 11 that with thedecrease of oscillating frequency, the value of Q2/Q1 increases whichmeans that the detection distance of the detection coil is reduced. Thevalue of C can be increased by adding an additional capacitor inparallel to the capacitor in FIG. 2. With this technique a detectioncoil or coil array can be used to sense foreign objects of differentsizes and/or at different distances in the z-direction.

It must be noted that the described FOD coil or coil arrayconfigurations are not only applicable to the detection method usingdecay in resonant circuit. Indeed, for any other detection method whichdetects the power loss or power dissipation in the foreign objects, suchFOD coil or coil array can be used, and they always have the advantageof separating friendly parasitic components and foreign objects.

Impact of Receiver Circuit

In another aspect of embodiments described herein, the impact of areceiver circuit on foreign object detection is considered. As describedabove, in many wireless power transfer systems, there is communicationor identification sent from a receiver to a transmitter to check forcompatibility between the charging device and the device being charged.If the two are compatible, power is transferred. This means that theabove foreign object detection takes place when a compatible receiver ispresent. This also means that the foreign object detection methodconsiders the power loss in the receiver circuits.

FIG. 12(a) shows a circuit diagram of a receiver circuit used in areceiver. The receiver circuit 1200 includes a power receiver coil 1204,a rectifier 1208 formed by diodes 1212A-D (collectively, 1212), acapacitor 1216, and a load 1220.

The power receiver coil 1204 receives AC power from a power transmittercoil (not shown). Through an optional resonant tank (not shown), thereceived power is transformed into DC power by the rectifier 1208 formedby the diodes 1212. The rectified power is provided to the load 1220.The receiver circuit 1200 is not restricted to receiving power from thetransmitter coil, but also from foreign object detection coils duringexecution of the foreign object detection process. If the receiver 1200consumes a measurable amount of power from the foreign object detectioncoil, the foreign object detection execution can incorrectly concludethat a foreign object is present. In one embodiment, the amplitude ofthe oscillation in the detection resonant circuit in FIG. 2 is limitedto a very low value, so that the coupled voltage in the receiver 1204 islower than the forward voltage of the diodes 1212 in the rectifier 1208.Once the coupled voltage is low, there is approximately no (or at leasta negligible or non-noticeable) power flow into the load capacitor 1216(having capacitance C_(dc)) and the load 1220. Limiting the oscillationamplitude in the detection resonant circuit can be done by, for example,reducing the initial voltage of the capacitor 208, V_(c) (see FIG. 2),to an adequate value. In another embodiment, the oscillating frequencyof the detection resonant circuit can be adjusted such that theoscillating frequency is far away from the resonant frequency of thereceiver. This can further limit the power captured by the receiver 1200from the foreign object detection coil.

Although it can be assumed in the above case that the diodes 1212 do notconduct electricity if the induced voltage in the receiver coil 1204 islower than the forward voltage of the diodes, the leakage energy in thereceiver circuit might still impact the result of the foreign objectdetection process. As shown in FIG. 12(b), the main contributor ofleakage is the parasitic capacitance of the diodes 1212, as marked asC_(d1), C_(d2), C_(d3) and C_(d4).

FIG. 13(a) shows the oscillation of the foreign object detectionresonant circuit (see FIG. 2) if there is no foreign object, no friendlyparasitic components, and no receiver. FIG. 13(b) shows the result whenan “uncharged” receiver is present. An “uncharged” receiver means thatthe initial voltage of C_(dc) in the receiver is equal or close to zero.By comparing the oscillation in FIG. 13(a) and FIG. 13(b), it is clearlyseen that the oscillation amplitude in FIG. 13(b) decays much morequickly and the oscillation duration in FIG. 13(b) is shorter. Theexecution of the foreign object detection process may therefore falselyregard the receiver as a foreign object. FIG. 13(b) also shows thecurrent flowing in C_(dc) and the voltage across it during the foreignobject detection process. It can be seen that some current does flowthrough the capacitor and charge it. This consumes the energy comingfrom the foreign object detection coil.

One embodiment to solve this issue is to “pre-charge” the receiverbefore the foreign object detection process starts. Such “pre-charge”can be done by injecting some power signal into the appropriatetransmitter coil(s) for a short period. With the coupling between theappropriate power transmitter coil(s) and the receiver, some energy istransferred to the receiver, and pre-charges the receiver. In particularit will pre-charge the capacitor C_(dc). FIG. 13(c) shows the resultingwaveform when the receiver is pre-charged. By comparing FIG. 13(a) andFIG. 13(c), it is seen that the oscillation waveform between them is thesame, and the receiver will not be falsely detected as a foreign object.FIG. 13(c) also shows the current flowing through C_(dc) and the voltageacross it. Compared to the current and voltage waveform shown in FIG.13(b), there is almost no current and voltage change in FIG. 13(c).

It should be noted that the described ‘pre-charge’ is not onlyapplicable to the detection method using decay in a resonant circuit.Indeed, for any other detection method which detects the power loss orpower dissipation in the foreign objects, such ‘pre-charge’ can be used,and with the advantage of separating the power loss in foreign objectsand receiver circuit.

The described foreign object detection approach is operated preferablywhen there is no power being transferred from transmitter to receiver(or vice versa), because the proposed foreign object detection processmay have difficulty to differentiate between the transferred power andthe power dissipation in a foreign object. One approach is totemporarily suspend the power transfer during execution of the foreignobject detection process.

For example, either the transmitter or the receiver could request suchtimeout by sending a specific command. This could be, for example, apower interruption packet in which the device indicates that thetransmitter should temporarily suspend power transfer for a specifictime period (which can be part of the command). The resulting timeoutcan then be used to perform one or more cycles of the foreign objectdetection process. This power interruption can be very short such that abuffer capacitor on a receiver has enough stored energy to bridge thepower transfer suspension, and no restart of the power transfer isneeded.

It should be noted that although the use of the example powerinterruption packet is described in embodiments specifically for foreignobject detection, it is not limited to this application. Another exampleis that the receiver may want to use a near field communication (“NFC”)circuit, which might not work during wireless power transfer. Byrequesting a short suspension of power transfer, the near fieldcommunication circuit function can be done during this period.

FIG. 14 shows a flow chart of an exemplary method 1400 for a foreignobject detection (“FOD” in the figure) algorithm. A transmitter mayoptionally begin the FOD process 1400 from an idle state 1404. Thesystem can be “woken up” 1408 (using methods such as capacitive sensing,motion sensing or pulsing sensing, etc.), thereby leaving the idle state1404. After the transmitter is woken 1408, it identifies 1412 acompatible receiver and finds the location of the receiver 1416. Inexamples in which the transmitter lacks multiple transmitter coils,digital receiver localization 1416 can be skipped.

As described above, once the compatible receiver is detected, it can bepre-charged 1420 with one or more power transmitter coils, asappropriate to the relative location and compatibility of the powerreceiver coils and power transmitter coils. Foreign object detectionprocess is executed 1424 with the previously identified one or moreappropriate FOD coils. If a foreign object is detected, optional warningfeedback 1432 can be given to the user. Otherwise, the transmitterbegins transferring power 1436 using the previously identifiedtransmitter coils that are in an appropriate location and of anappropriate type to transfer power to the receiver. During the transferof power 1436, if a power interruption packet is received 1440 (or,alternatively, when the transmitter decides to initiate an additionalFOD execution), the transmitter executes the pre-charge of receiver1420, and does an additional FOD execution again. Optionally, thepre-charge 1420 need not be performed when power transfer 1436is alreadyin progress. Instead, the process can proceed from receiving the powerinterruption packet 1440 to executing the foreign object detectionprocess 1424, as shown by a dashed arrow. It should be noted that, forsimplicity, the flow chart in FIG. 14 does not show a step indicatingthe end of the power transfer process.

Although all the proposed methods and improvements in this disclosurecan be used together to achieve accurate foreign object detection in awireless power transfer system, they may also be used individually or incombination.

Miscellaneous

The foregoing description of the embodiments of the invention has beenpresented for the purpose of illustration; it is not intended to beexhaustive or to limit the invention to the precise forms disclosed.Persons skilled in the relevant art can appreciate that manymodifications and variations are possible in light of the abovedisclosure.

Some portions of this description describe the embodiments of theinvention in terms of algorithms and symbolic representations ofoperations on information. These algorithmic descriptions andrepresentations are commonly used by those skilled in the dataprocessing arts to convey the substance of their work effectively toothers skilled in the art. These operations, while describedfunctionally, computationally, or logically, are understood to beimplemented by computer programs or equivalent electrical circuits,microcode, or the like. Furthermore, it has also proven convenient attimes, to refer to these arrangements of operations as modules, withoutloss of generality. The described operations and their associatedmodules may be embodied in software, firmware, hardware, or anycombination thereof.

Any of the steps, operations, or processes described herein may beperformed or implemented with one or more hardware or software modules,alone or in combination with other devices. In one embodiment, asoftware module is implemented with a computer program productcomprising a computer-readable medium containing computer program code,which can be executed by a computer processor for performing any or allof the steps, operations, or processes described.

Embodiments of the invention may also relate to an apparatus forperforming the operations herein. This apparatus may be speciallyconstructed for these operations, and/or it may comprise ageneral-purpose computing device selectively activated or reconfiguredby a computer program stored in the computer. Such a computer programmay be stored in a non-transitory, tangible computer readable storagemedium, or any type of media suitable for storing electronicinstructions, which may be coupled to a computer system bus.Furthermore, any computing systems referred to in the specification mayinclude a single processor or may be architectures employing multipleprocessor designs for increased computing capability.

Embodiments of the invention may also relate to a product that isproduced by a computing process described herein. Such a product maycomprise information resulting from a computing process, where theinformation is stored on a non-transitory, tangible computer readablestorage medium and may include any embodiment of a computer programproduct or other data combination described herein.

Finally, the language used in the specification has been principallyselected for readability and instructional purposes, and it may not havebeen selected to delineate or circumscribe the inventive subject matter.It is therefore intended that the scope of the invention be limited notby this detailed description, but rather by any claims that issue on anapplication based hereon. Accordingly, the disclosure of the embodimentsof the invention is intended to be illustrative, but not limiting, ofthe scope of the invention, which is set forth in the following claims.

What is claimed is:
 1. A wireless power charging system for charging aseparate device having a power receiver coil, the wireless powercharging system comprising: a power transmitter coil; and a foreignobject sensor that includes a resonant tank circuit formed by a foreignobject detection (FOD) coil coupled to a capacitor, wherein the FOD coilhas a smaller lateral area than the power transmitter coil; wherein: thepower transmitter coil pre-charges the power receiver coil in theseparate device; the foreign object sensor determines a quality factorQ2 of the resonant tank circuit while the power receiver coil ispre-charged; the foreign object sensor determines whether a foreignobject is present, based on a comparison of the quality factor Q2 with aquality factor Q1 of the resonant tank circuit when no foreign object ispresent; and the power transmitter coil transmits power by inductivecoupling to the power receiver coil in the separate device, if theforeign object sensor determines that no foreign object is present. 2.The system of claim 1, wherein the foreign object sensor determines thata foreign object is present if the quality factor Q2 is less than thequality factor Q1.
 3. The system of claim 1, wherein the foreign objectsensor further comprises: detection circuitry coupled to the resonanttank circuit, the detection circuitry configured to determine thequality factor of the resonant tank circuit.
 4. The system of claim 3,wherein the detection circuitry determines the quality factor of theresonant tank circuit based on a decay of a response of the resonanttank circuit.
 5. The system of claim 3, wherein the detection circuitrycharges the capacitor and determines the quality factor of the resonanttank circuit based on a decay of a voltage across the capacitor.
 6. Thesystem of claim 1, wherein the FOD coil has a size similar to a size ofa target foreign object.
 7. The system of claim 1, wherein a lateraldimension of the FOD coil is not more than 25 mm.
 8. The system of claim1, wherein a lateral dimension of the FOD coil is approximately 20 mm.9. The system of claim 1, wherein the power transmitter coil and the FODcoil are each rectangular.
 10. The system of claim 1, wherein the powertransmitter coil and the FOD coil are each circular.
 11. The system ofclaim 1, wherein the power transmitter coil has a lateral area that isat least two times a lateral area of the FOD coil.
 12. The system ofclaim 1, wherein the FOD coil is laterally positioned within a perimeterof the power transmitter coil.
 13. The system of claim 12, wherein theFOD coil is centered relative to the power transmitter coil.
 14. Thesystem of claim 1, wherein the FOD coil is positioned between the powertransmitter coil and the power receiver coil.
 15. The system of claim 1,wherein the power transmitter coil is positioned between the FOD coiland the power receiver coil.
 16. A wireless power charging system forcharging a separate device having a power receiver coil, the wirelesspower charging system comprising: a plurality of power transmittercoils; and a foreign object sensor that includes a plurality of resonanttank circuits each formed by a foreign object detection (FOD) coilcoupled to a capacitor, wherein each FOD coil has a lateral area that issmaller than a lateral area of any of the power transmitter coils;wherein: one or more of the power transmitter coils pre-charge the powerreceiver coil in the separate device; the foreign object sensordetermines quality factors Q2 of some of the resonant tank circuitswhile the power receiver coil is pre-charged; for each of said resonanttank circuits, the foreign object sensor determines whether a foreignobject is present, based on a comparison of the quality factor Q2 forthat resonant tank circuit with a quality factor Q1 of that resonanttank circuit when no foreign object is present; and at least one powertransmitter coil transmits power by inductive coupling to the powerreceiver coil in the separate device, if the foreign object sensordetermines that no foreign object is disposed between that powertransmitter coil and the power receiver coil.
 17. The system of claim16, wherein the foreign object sensor determines that a foreign objectis present for a FOD coil if the quality factor Q2 that resonant tankcircuit is less than the quality factor Q1 for that resonant tankcircuit.
 18. The system of claim 16, wherein the foreign object sensordetermines the quality factor of each resonant tank circuit based on adecay of a response of each resonant tank circuit.
 19. The system ofclaim 16, further comprising: a controller configured to select whichpower transmitter coils to transmit power, based on which resonant tankcircuits have detected no foreign object.
 20. The system of claim 16,further comprising: a controller configured to select which FOD coils toactivate, based on which power transmitter coils are transmitting power.